timestamps.md

Timestamps

Time format	Example of where used	Explanation of format	Type (flatbuffers)
Nanoseconds past epoch	Event schema	Nanoseconds since Unix epoch (1 Jan 1970), requires 64 bit integer	ulong
ISO 8601	NeXus files	https://en.wikipedia.org/wiki/ISO\_8601, UTC for NeXus	string
EPICS timestamp	EPICS PVs	Struct of seconds since 1 Jan 1990 and nanoseconds past that	stuct (ulong, int)

Example conversions

nanoseconds past epoch to ISO 8601

Python:

def nanoseconds_to_iso8601(nanoseconds):
    dt = datetime.datetime.fromtimestamp(nanoseconds / 1e9)
    return '{}{:03.0f}'.format(dt.strftime('%Y-%m-%dT%H:%M:%SZ%f'), nanoseconds % 1e3)

EPICS timestamp to nanoseconds past epoch

C++:

uint64_t epicsToNanosecsPastEpoch(uint64_t epicsSeconds, int32_t epicsNanoseconds) {
  // You must be explicit about type here (hence the "L" suffix) or C++ will happily
  // convert your constants to double, which will mess up the calculation.
  std::uint64_t unixSeconds = epicsSeconds + 631152000L;
  std::uint64_t nanosecondsPastEpoch = (unixSeconds * 1000000000L) + epicsNanoseconds;
  return nanosecondsPastEpoch;
}